A light emitting diode (LED) is a semiconductor device constituted mainly by group III-V compound semiconductor materials, for instance. Such semiconductor materials have a characteristic of converting electricity into light. Hence, when a current is applied to the semiconductor materials, electrons therein are combined with holes, and excessive energy is released in form of light, thereby achieving an effect of luminosity.
A sapphire substrate is often used as in a blue-green LED which dominates the market of liquid crystal displays and lighting tools, and an active layer of such LED is made of Gan/InGaN materials epitaxially grown on the sapphire substrate. Due to the mismatched lattice constant of the epitaxially grown GaN/InGaN film and the sapphire substrate, the resultant heteroepitaxy frequently leads to high defect density (approximately 109 cm−2 to 1010 cm−2) in the nitride film; in addition, insufficient thermal conductivity (0.3 W/cmK) often results in the significant difference in thermal expansion coefficients. As a result, the residual stress often causes distortion or cracks of chips, thus posing a negative impact on the LED with high luminous efficiency.
Moreover, because of the difference between the indices of refraction of GaN and air, merely 4.54% of light generated in the LED can be successfully emitted from the GaN surface to the air. Therefore, the low light extraction efficiency causes drastic reduction in the external quantum efficiency of the LED, and accordingly the industry is earnestly pursuing an LED with high light extraction efficiency.